Coronal Observations

Observations are made at the National Solar Observatory at Sacramento Peak with the Fisher-Smartt Emission Line Coronal Photometer (ELCP). This instrument photoelectrically records the solar corona when fed with the John W. Evans Solar Facility 40-cm-aperture Coronagraph. It operates at high precision due to its ability to subtract the sky background from the signal in emission lines through use of a lockin amplifier oscillating at a rate of 100 kHz between the continuum and lines at 637.4 nm (Fe X), 530.3 nm (Fe XIV) and 569.4 nm (Ca XV), which are formed at approximate temperatures of 1, 2 and 3 MK, respectively. A 1.1 arcmin aperture is scanned around the limb daily from 1.15 to 1.45 solar radii (Ro) for Fe X and Fe XIV, and 1.15 solar radii for Ca XV. The output of the ELCP is sensed by a photomultiplier, digitized and recorded every 3 degrees of latitude. Absolute intensities in millionths of the brightness of the center of the disk at each wavelength are obtained by calibrating the system through a neutral density filter. All Fe X and Ca XV scans are adjusted to have at least one absolute zero intensity data point. The lower-height Fe XIV scans are adjusted for zero level by determining the zero level of an upper scans (1.45 or 1.35 Ro) and subtracting that amount from the lower scans.

The pseudo-full-disk maps are produced by joining together 14 days of data and projecting it onto a sphere. The most recent scan is on the left of the map, and the data on the central meridian are from 7 days prior to the date of the map. Data are incremented from the central meridian at 12.857 degrees per day. Missing data are interpolated. West-limb maps, which show the farside of the Sun on the day they are produced, have been given an effective date two weeks into the future, so that they may be compared with East-limb maps of the same date. Maps are currently produced for Fe XIV and Ca XV and are normally available for each Monday through Friday, excluding holidays.